1. Field of the Invention
The present invention relates to a power converter with a neutral point clamped (NPC) circuit having a bidirectional switch interposed between a power supply and a series connection point of a pair of semiconductor switching elements.
2. Description of the Related Art
An inverter ON/OFF-drives a pair of series-connected semiconductor switching elements in a certain relation between the two to switch a DC voltage and is known for a small-sized and highly efficient power converter. Of the inverters, a multi-level inverter, typically a three level inverter, has many advantages for achieving a small-sized and highly efficient inverter over a common two-level inverter. The three level inverter is also called a neutral point clamped (NPC) inverter. The output voltages at the neutral point are three values of ±Vin/2, and 0 (zero), where Vin is the DC voltage. Consequently, output voltages of a three phase inverter using the three level inverter are five values of ±Vin, ±Vin/2, and 0. These values generate an AC output voltage having a nearly sinusoidal waveform. Multi-level inverters are disclosed in Patent Documents 1 and 2, for example.
FIG. 1 shows a schematic construction of an inverter with an advanced NPC circuit constructing a three-phase AC power converter. This inverter, which is a power converter, comprises three pairs of, i.e., six semiconductor switching elements Q1 thorough Q6, which can be IGBTs, for example. The semiconductor switching elements Q1 through Q6 form three sets of half-bridge circuits each set composed of a pair of series-connected semiconductor switching elements, and ON/OFF driven in a certain relationship with each other to switch the DC voltage Vin. These half-bridge circuits are connected in parallel to construct a three-phase AC full-bridge circuit. Each of the semiconductor switching elements Q1 through Q6 is provided with a respective one of free-wheeling diodes D1 through D6 connected in anti-parallel to the switching element. Between a power supply and a middle point of each half-bridge, each of three bidirectional switches S1 S2, and S3 is connected to clamp the semiconductor switching elements Q1 through Q6 to the neutral point, which is a point of middle potential Vin/2, of the DC voltage Vin.
These bidirectional switches S1 S2, and S3 are, as shown in FIG. 2A, for example, composed of anti-parallel-connected switching elements Q11 and Q12, which can be IGBTs, and diodes D11 and D12 connected in series to the respective switching elements Q11 and Q12. These diodes D11 and D12 assist a reverse withstand voltage of the diodes D11 and D12. Alternatively in recent many cases, the bidirectional switches S1, S2, and S3 are composed of switching elements Q21 and Q22 with an enhanced reverse withstand voltage, for example, reverse-blocking IGBTs, connected in anti-parallel as shown in FIG. 2B. This construction is disclosed in Patent Documents 3 and 4, for example.
The semiconductor switching elements Q1 through Q6 can be MOS-FETs as well as the IGBTs. The switches S1, S2 and S3 can be constructed with reverse-blocking MOS-FETs as well as with the reverse-blocking IGBTs.